Certain impact-modified plastics, such as styrene-maleic anhydride copolymers (SMA's), high impact polystyrenes (HIPS), methylmethacrylate-butadiene-styrene copolymer, transparent impact polystyrene, and acrylonitrile butadiene styrene copolymer, are typically prepared in the presence of a rubber; for example, a polybutadiene or a styrene-butadiene copolymer, to enhance toughness, impact strength and other properties. A low solution viscosity additive rubber can facilitate the dispersion of the rubber in the plastic phase. Moreover, early in polymerization of HIPS, phase separation begins because of the immiscibility of the rubber within the polystyrene being formed and depletion of the styrene phase. Furthermore, in SMAs, a low solution viscosity may improve the clarity and the gloss of the resultant product.
While low solution viscosity of the additive rubber is highly desirable, it makes commercial handling difficult. For example, low solution viscosity typically leads to a liquid or semi-liquid material that is difficult to package and ship. Accordingly, a relatively high bulk viscosity material capable of being baled into a shippable and easy to handle form is desirable.
The commonly assigned U.S. Pat. No. 7,105,613, which is hereby incorporated by reference in its entirety, discloses a polymeization technique for carboxylate termination of rubber polymers to achieve a high bulk viscosity, low solution viscosity rubber compound. This produces a baleable rubber compound that is useful as an additive in plastics.
Yet there remains a need for further improved polymers for use in impact-modified plastics; for example, polymers that contribute to plastics to provide improved Izod impact values, and polymers that may be used in decreased amounts to affect the impact strength of plastic compositions. There is also a need for an additive that can reduce the solution viscosity of the polymer while maintaining a high bulk viscosity so the polymer remains baleable. An additive that can facilitate control of gloss and Izod strength is also desirable.